The invention relates to crosslinkable polymer powder compositions, to processes for preparing them, and to the use thereof.
Crosslinkable dispersion powders are known from U.S. Pat. No. 3,409,578 in which powder compositions comprising carboxyl-containing polymers and/or carboxyl-containing protective colloids are crosslinked in the presence of polyvalent metal ions. A disadvantage is that such compositions containing polyvalent metal salts frequently tend to release acid in the presence of water and to undergo premature crosslinking, especially on contact with moisture. EP-B 687317 describes water redispersible, crosslinkable powders based on N-methylol-functional copolymers which crosslink thermally. A disadvantage of the latter powder is the fact that powders which can be crosslinked by means of heat alone are unsuitable for use in many applications.
EP-A 721004 discloses crosslinkable, redispersible powder mixtures comprising film-forming polymer particles containing at least one functional, crosslinkable group. The crosslinkable group may also be introduced by way of the protective colloids. For crosslinking, this document recommends the addition of an external crosslinker comprising at least one reactive component which enters into nonionic bonds with the functional groups when the mixture is redispersed in water. Boron compounds are cited as an example.
It is known that boric acid and borax cause thickening and gelling of polyhydroxy compounds such as polyvinyl alcohol, cellulose, and dextran, and may therefore gel to form inclusion compounds with herbicides, for example, in the strongly alkaline range (EP-A 88552).
Since the crosslinking reaction proceeds spontaneously and, in contrast to salt coagulation, is irreversible by heating, the practical possibilities for application are very restricted. For example, a redispersible powder stabilized with polyvinyl alcohol cannot be redispersed in the presence of boric acid or borax, in an amount sufficient to gel the polyvinyl alcohol, in an aqueous medium whose pH is more than 8.
An object of the invention is therefore to provide crosslinkable polymer powder compositions which crosslink, while ensuring an appropriate processing time, even in an alkaline aqueous medium and even at room temperature.